DC switching power supplies are vital components of electronic circuits. They are powered by a supply voltage and provide output voltages at various levels to power the electronic circuits. To produce these various output voltage levels, DC switching power supplies include DC-to-DC converters containing transformers. The primary and secondary windings of the transformer are not perfectly coupled to each other or to the magnetic material within the transformer. This imperfect coupling results in leakage energy in the magnetic characteristics of the transformer that builds up over time and is stored within the transformer. Subsequently, the stored leakage energy causes the transformer to operate as if inductors are connected in series with the windings, with the inductors storing the leakage energy. This is known as leakage inductance. Unless dissipated, the stored leakage energy will negatively impact the performance of the power supply. If the leakage energy is not dissipated, it may result in voltages and/or currents above the design specifications of the power supply components resulting in damage to the components or unsafe operating conditions.
To prevent the build up of the leakage energy, power supplies typically either dissipate the leakage energy in a resistor, or feed the leakage energy back to the DC input voltage source at the input to the transformer. The leakage energy is fed back by using a separate winding coupled to the primary winding of the transformer. The first approach is wasteful because the energy is dissipated. The second approach results in additional leakage energy from the separate winding that builds up if not dissipated. Also, power supplies typically contain circuitry that requires power from voltages higher than the DC input voltage source. In addition, power supplies typically have hold-up capacitors charged to a higher voltage than the DC input voltage source. These hold-up capacitors provide energy to the power supply circuitry during a momentary loss in the DC input voltage source. Obtaining these higher voltages requires additional circuitry such as additional transformer windings or additional DC-to-DC converter circuitry. Such additional circuitry is complicated, consumes space, adds weight, increases the cost of the power supply, and results in an inefficient use of power.